A heretofore known cool mug, referring to FIG. 1, is provided which has two rims 10, 11, forming a closed chamber in which water is held.
The cool mug 1 of the prior art is provided to keep drinks held therein cold and is initially kept in a freezer so that the water held in the closed chamber becomes ice. According to experience, approximately four hours is needed for water to freeze to be ready for use.
Considering the fact that water expands when it freezes and that it takes a relatively long time for water to freeze, only part of the closed chamber is used to hold the water.
From the above discussion, it can be understood that the cool mug 1 has disadvantages as follows:
1. The four hours needed for water to freeze is generally too long to meet users' needs; and, PA1 2. The low temperature keeping effect of the cool mug 1 is generally available for only one hour.